The invention relates to a chip resistor comprising a cuboid resistor body of ceramic material and solderable, metal, current-supply strips at a first pair of opposite side faces of the resistor body.
The invention also relates to a method of manufacturing a chip resistor, in which a cuboid resistor body is provided at two opposite side faces with metal, current-supply strips.
The invention can particularly suitably be applied to resistors having no lead wires, a semi-conductive ceramic material being used as a resistance material, in particular materials having a negative (NTC) or a high positive (PTC) temperature coefficient of electrical resistance.
U.S. Pat. No. 3,027,529 describes a PTC resistor, in which a resistor body in the form of a cylinder or a disc is used. The electric connections consist of metal caps which are fitted around the ends of the cylinder, or of lead wires which are soldered to the flat sides of the disc.
In the manufacture of electric components having no lead wires, the dimensions of which should be as small as possible, and which should be manufactured in large numbers at low costs, the application of caps is undesired in many cases. According to an alternative method, contact faces for the supply of electric current are manufactured by means of sputtering, metal spray or vapour deposition, but then it is not easy to manufacture contact faces which extend around the edges of the component.
Components having no lead wires, which are preferably cuboid, should at each end be provided with terminals on three faces owing to the various soldering techniques used for mounting on a printed circuit board. In the case of wave soldering, a component is temporarily fixed to a printed circuit board by means of an adhesive, after which a solder wave is led over the surface of the board. This technique requires the presence of terminals at the side faces of the electric component. In a vapour soldering process, drops of a solder paste are placed on the printed circuit board, after which the electric components are provided and the assembly is heated in a vapour, the solder paste being converted into a conductive contact material. This technique requires the presence of terminals on the lower side of the electric component which lies against the printed circuit board. For reasons of symmetry there is preferably also a terminal on the upper side so as to render an additional check superfluous when the electric component is mounted on the printed circuit board.
The non-prepublished Netherlands Patent Application NL-A-8800156 in the name of Applicants relates to a chip resistor as described in the opening paragraph, in which a second pair of opposite side faces is covered completely with electrically insulating layers, and in which the solderable metal strips are directly connected, both mechanically and electrically conductively, to the resistor body. The chip resistor is manufactured from a plate of ceramic resistance material which is divided into strips and which strips are subsequently divided into cuboids.
In chip resistors the spacing between the contact faces is preferably as large as possible with a view to the positional accuracy on a printed circuit board. During operation of the chip resistor the electric current passes through a block of resistance material of small cross-sectional area over a substantial length. For this reason, this construction is particularly suitable for the manufacture of a chip resistor having a large resistance value. It is not easy to manufacture a chip resistor having a relatively small resistance value with sufficient accuracy. The resistors can be measured and sorted after the manufacture, but in the case of a tolerance of, for example, less than 1% the number of rejects will be high.